The present invention relates to an optical spatial modulation device for modulating radiated light in accordance with information representing an image to be displayed for each pixel and an image display apparatus employing the optical spatial modulation device.
A liquid crystal display device is known as an optical spatial modulation device for modulating radiated light in accordance with information representing an image to be displayed for each pixel. There has been widely used a ordinary liquid crystal display device of a type wherein the intensity of light is modulated by putting an array of so-called twisted nematic liquid crystals, that is, liquid crystals used in a twisted nematic operating mode, in continuously varying states. A twisted nematic liquid crystal is referred to hereafter simply as a TN liquid crystal.
With regard to such liquid crystal materials and liquid crystal display devices, there have been published an article with a title "New Technology Seen from Patent Information: 13 Liquid Crystal," authored by Yoko Watanabe, Pages 26 to 31, Volume 92, a July 1995 issue of "Inventions", a monthly magazine published by Japan Institute of Invention and Innovation, an article with a title "New Technology Seen from Patent Information: 26 Liquid Crystal Display Device," authored by Takashi Hinatsu, Pages 62 to 69, Volume 92, an August 1996 issue of "Inventions", a monthly magazine published by Japan Institute of Invention and Innovation and a book with a title "Liquid Crystal Device Handbook," issued by the 142.sup.nd Committee of the Japan Society for the Promotion of Science and published by the Nikkan Kogyo Newspaper.
However, the TN liquid crystal has a problem of a slow response speed, making it desirable to develop an optical spatial modulation device that is capable of operating at a high speed. As a liquid crystal material for light modulation that is capable of operating at a high speed, for example, there are a ferroelectric liquid crystal referred to hereafter as an FLC and a non-ferroelectric inductive liquid crystal. An FLC has a state storing characteristic. In general, as a state, there may be only 2 values. Thus, with an optical spatial modulation device using such a material for light modulation, continuous light modulation can not be carried out. That is to say, such a device is capable of operating by merely changing from an on state to an off state or vice versa.
In addition, as a liquid crystal material for light modulation having a state storing characteristic, there are also a cholesteric liquid crystal or a kailar nematic liquid crystal operating in a phase transition mode and a polymer liquid crystal which carries out write and erase operations between isotropic and glass phases. Other liquid crystal materials that can be used as a material for light modulation include a polymer diffusion liquid crystal which is referred to hereafter as a PDLC.
When displaying a multi-tone image by means of an optical spatial modulation device using such a material for light modulation, a pulse width modulation (PWM) technique is typically adopted in order to make use an afterglow characteristic of the human eye. To put it in detail, when displaying a multi-tone image by means of such an optical spatial modulation device, light is switched from an on state to an off state and vice versa at a high speed with controlled timing so that the multi-tone image is displayed and projected on a human eye.
The following is a description of an image display apparatus employing such an optical spatial modulation device.
FIG. 22 is a conceptual diagram showing the image display apparatus. As shown in the figure, a light generated by a light source 101 is radiated to an optical spatial modulation device 103 by a radiation optical system 102. The light modulated by the optical spatial modulation device 103 is then projected on a screen 105 by a projection optical system 104. As a result, an image is displayed on the screen 105.
FIG. 23 is a diagram showing a squint view of disassembled components of an enlarged portion of the optical spatial modulation device 103 mentioned above. As shown in the figure, the optical spatial modulation device 103 comprises a driving layer 106, a reflection layer 107, a modulation layer 108 and a common electrode 109. It should be noted that, if the modulation layer 108 is implemented by a crystal, an orientation layer is provided between the common electrode 109 and the crystal and another orientation layer is provided between the crystal and the reflection layer 107.
In an operation to drive this optical spatial modulation device 103, first of all, data coming from a data line 111 is written into each memory cell 112 at a point of intersection of a scanning line 110 and the data line 111 created on the driving layer 106. Each of the memory cells 112 corresponds to a pixel.
Next, electric fields are applied to the electrically charged modulation layer 108 provided between reflection pads 113 created on the reflection layer 107 and the common electrode 109 in accordance with pieces of data recorded in the memory cells 112 so as to reflect the respective pixels. The modulation layer is implemented typically by an FLC. As a result, areas of the modulation layer 108 are put in either a light passing state or a light shielding state in dependence on pixels facing the areas.
Then, some of the light radiated to the optical spatial modulation device 103 passing through the modulation layer 108 is reflected by the reflection pads 113 on the reflection layer 107 and output by way of the reflection layer 108 as shown in FIG. 22. That is to say, only some of the light radiated to the optical spatial modulation device 103 that manages to pass through the modulation layer 108 is reflected. As a result, light is modulated for each pixel.
In order to continuously change a displayed image in this image display apparatus, the radiation of a light from the light source 101 is halted each time the image is changed and then the state of the modulation layer 108 is changed with respect to all the pixels. Then, at a point of time the operation to change the state of the modulation layer 108 with respect to all the pixels is completed, the radiation of the light from the light source 101 is resumed. As a result, lights modulated for the pixels are sequentially projected on the screen 105. Thus, while the state of the modulation layer 108 of the optical spatial modulation device 103 employed in the image display apparatus is being changed, the light source 101 is turned off. As the operation to change the state of the modulation layer 108 is completed, a light is radiated from the light source 101 to the optical spatial modulation device 103.
It should be noted that, since the FLC normally has a state storing characteristic, once an electric field is applied to put the FLC in a desired state, residual electric charge remains in the FLC. It is thus necessary to apply an electric field to the FLC in the opposite direction in order to neutralize the residual electric charge. As a technique for neutralizing residual electric charge, among other methods, a 2-field technique is known. In the 2-field technique, pieces of pixel data of a desired image are written into the memory cells 112 in order to apply electric fields to the modulation layer 108 in accordance with the pieces of pixel data to display the desired image. Then, pieces of pixel data for inverting the pieces of pixel data of the displayed image are written into the memory cells 112 in order to apply electric fields to the modulation layer 108 in accordance with the pieces of pixel data newly written into the memory cells 112. That is to say, according to this 2-field technique, residual electric charge is neutralized by alternately applying electric fields to the modulation layer 108 in opposite directions for a display of 1 picture.
The image display apparatus like the one described above is further explained by referring to timing charts shown in FIG. 24. In the example shown in the figure, the 2-field technique is adopted. In this case, the number of scanning lines 110 is n.
As shown in FIG. 24, a period of time required for displaying 1 screen comprises an uninverted data write period and an inverted data write period. During the uninverted data write period, pieces of pixel data of a desired image are written into the memory cells 112 in order to apply electric fields to the modulation layer 108 in accordance with the pieces of pixel data to display the desired image. During the inverted data write period, on the other hand, pieces of pixel data for neutralizing the pieces of pixel data of the displayed image are written into the memory cells 112 in order to apply electric fields to the modulation layer 108 in a direction opposite to the electric fields applied during the uninverted data write period.
The uninverted data write period comprises a data write period and a light emit period. During the data write period, pixel data of an image to be displayed is written into memory cells 112 in order to put the modulation layer 108 in a predetermined state corresponding to the image to be displayed. During the light emit period, on the other hand, a light is radiated from the light source 101 to the optical spatial modulation device 103 wherein the modulation layer 108 is put in a predetermined state corresponding to the image to be displayed. That is to say, only during the light emit period is the image actually displayed.
On the other hand, the inverted data write period comprises an inverted data write period and a light emit equivalent period. During the inverted data write period, pixel data for inverting the pixel data of the image displayed during the light emit period is written into memory cells 112 in order to put the modulation layer 108 in an inverted state. The light emit equivalent period is a period required to make the length of the inverted data write period equal to the length of the uninverted data write period so that residual electric charge is completely neutralized. During the light emit equivalent period which is a counterpart of the light emit period, the modulation layer 108 is held in an inverted state.
During the data write period, pixel data generated along the scanning lines 110 is supplied to the data lines 111 and written into the memory cells 112.
To put it in detail, first of all, pixel data D1 is supplied to the data lines 111 and, at the same time, a write signal is supplied to the 1.sup.st scanning line 110 during the data write period. Thus, the pixel data D1 is written into the memory cells 112 connected to the 1.sup.st scanning line. It should be noted that the pixel data D1 is part of pixel data of an image to be displayed, that is, pieces of data for pixels corresponding to the memory cells 112 connected to the 1.sup.st scanning line. Then, areas of the modulation layer 108 facing the memory cells 112 on the 1.sup.st scanning line which correspond to the pixels are put in a light passing state or a light shielding state in dependence on the pixel data D1. That is to say, the states of pixels corresponding to the memory cells 112 connected to the 1.sup.st scanning line are set depending on the pixel data D1.
Then, pixel data D2 is supplied to the data lines 111 and, at the same time, a write signal is supplied to the 2.sup.nd scanning line 110. Thus, the pixel data D2 is written into the memory cells 112 connected to the 2.sup.nd scanning line. It should be noted that the pixel data D2 is part of pixel data of an image to be displayed, that is, pieces of data for pixels corresponding to the memory cells 112 connected to the 2.sup.nd scanning line. Then, areas of the modulation layer 108 facing the memory cells 112 on the 2.sup.nd scanning line which correspond to the pixels are put in a light passing state or a light shielding state in dependence on the pixel data D2. That is to say, the states of pixels corresponding to the memory cells 112 connected to the 2.sup.nd scanning line are set depending on the pixel data D2.
Thereafter, pixel data is written into the memory cells 112 connected to subsequent scanning lines and the states of pixels corresponding to the memory cells 112 connected to the scanning lines are set depending on the pixel data in the same way. Finally, pixel data Dn is supplied to the data lines 111 and, at the same time, a write signal is supplied to the n.sup.th scanning line 110. Thus, the pixel data Dn is written into the memory cells 112 connected to the n.sup.th scanning line. It should be noted that the pixel data Dn is part of pixel data of an image to be displayed, that is, pieces of data for pixels corresponding to the memory cells 112 connected to the n.sup.th scanning line. Then, areas of the modulation layer 108 facing the memory cells 112 on the n.sup.th scanning line which correspond to the pixels are put in a light passing state or a light shielding state in dependence on the pixel data Dn. That is to say, the states of pixels corresponding to the memory cells 112 connected to the n.sup.th scanning line are set depending on the pixel data Dn.
As described above, all the pixels are set in states reflecting the image to be displayed during the data write period. It should be noted that, during the data write period, the light source 101 is turned off in order to avoid a disorder state of a light reflected in a state transition of the modulation layer 108.
By the same token, first of all, pixel data D1' is supplied to the data lines 111 and, at the same time, a write signal is supplied to the 1.sup.st scanning line 110 during the inverted data write period. Thus, the pixel data D1' is written into the memory cells 112 connected to the 1.sup.st scanning line. It should be noted that the pixel data D1' is part of pixel data of an image to be displayed, that is, pieces of data for pixels corresponding to the memory cells 112 connected to the 1.sup.st scanning line. Then, the states of areas of the modulation layer 108 facing the memory cells 112 on the 1.sup.st scanning line which correspond to the pixels are inverted in dependence on the pixel data D1'.
Then, pixel data D2' is supplied to the data lines 111 and, at the same time, a write signal is supplied to the 2.sup.nd scanning line 110. Thus, the pixel data D2' is written into the memory cells 112 connected to the 2.sup.nd scanning line. It should be noted that the pixel data D2' is part of pixel data of an image to be displayed, that is, pieces of data for pixels corresponding to the memory cells 112 connected to the 2.sup.nd scanning line. Then, the states of areas of the modulation layer 108 facing the memory cells 112 on the 2.sup.nd scanning line which correspond to the pixels are inverted in dependence on the pixel data D2'.
Thereafter, pixel data is written into the memory cells 112 connected to subsequent scanning lines and the states of pixels corresponding to the memory cells 112 connected to the scanning lines are set depending on the pixel data in the same way. Finally, pixel data Dn' is supplied to the data lines 111 and, at the same time, a write signal is supplied to the n.sup.th scanning line 110. Thus, the pixel data Dn' is written into the memory cells 112 connected to the n.sup.th scanning line. It should be noted that the pixel data Dn' is part of pixel data of an image to be displayed, that is, pieces of data for pixels corresponding to the memory cells 112 connected to the n.sup.th scanning line. Then, the states of areas of the modulation layer 108 facing the memory cells 112 on the n.sup.th scanning line which correspond to the pixels are put in a light passing state or a light shielding state in dependence on the pixel data Dn'.
As described above, the states of all the pixels are inverted during the inverted data write period. It should be noted that, during the data write period and the light emit equivalent period, the light source 101 is turned off.
As indicated by the timing charts shown in FIG. 24, in the ordinary liquid crystal display device, data stored in memory cells of each scanning line is rewritten and then, after data of all memory cells has been rewritten, light is radiated from a light source. Thus, in the case of a liquid crystal display device with a large number of scanning lines, the time allocated to emission of light is shortened and, as a result, a high intensity can not be obtained anymore.
In addition, even if an FLC having a response speed faster than a TN liquid crystal is used, the response speed of its optical spatial modulation device is not sufficient, making it desirable to further increase the response speed.
On the top of that, when a light modulation material that needs neutralization of residual electric charge is used, it is necessary to write inverted data after displaying an image as described above in order to neutralize electric charge injected into a modulation layer, or to supply a pulse voltage for neutralizing electric charge. However, a period for neutralizing electric charge does not contribute to the display of an image. As a result, such neutralization period causes saturation of the response speed and deterioration of the intensity. In the case of neutralization of electric charge by using the 2-field technique as shown in FIG. 24, for example, it is necessary to provide an inverted data write period in addition to the uninverted data write period. As a result, the frame rate is reduced by half, making the efficiency poorer.